1. Field of the Invention
The present invention relates to a high-voltage high-current circuit design; and, in particular, relates to the design of a driver circuit for a "high-side" switch.
2. Discussion of the Related Art
Transistors which operate under high voltages and deliver large currents are found in many applications. One such application is an implantable defibrillator, in which a large current is delivered to halt a ventricular fibrillation in a cardiac arrhythmia patient. In that application, a large voltage (e.g. up to 750 volts) is applied across the patient's heart, and two or more large current pulses (e.g. up to 50 A for 10 milliseconds (approx.) duration) of alternating polarities are provided to a patient suffering from ventricular fibrillation. For such a high-voltage high-current application, the output transistor delivering the current pulses can be provided, for example, by an insulated-gate bipolar transistor ("IGBT").
FIG. 1 shows a typical output circuit for such a defibrillator. As shown in FIG. 1, an "H-bridge" circuit 100 is provided to deliver currents in two directions (i.e. directions indicated by arrows 105 and 106 in FIG. 1) through a load 101. Load 101, in this application, is a human heart. H-bridge circuit 100 includes IGBT transistors 104a, 104b, 104c and 104d controlled respectively by gate driver circuits 102a, 102b, 103a and 103b. Gate driver circuits 102a and 102b are active to drive a current through transistors 104a and 104b in direction 105. Gate driver circuits 103a and 103b are active to drive a current through transistors 104c and 104d in direction 106.
In the prior art, each of gate driver circuits 102a, 102b, 103a and 103b can be implemented by a high current MOSFET driver, such as the HV400 High current MOSFET driver from Harris Corporation. FIG. 2 shows a unipolar gate driver output circuit 200 in the prior art, which includes a high current MOSFET driver integrated circuit 202 (e.g. HV400) driving the gate terminal of a high current MOSFET 201. Circuit 202 is, in turn, driven by the output terminals of an isolation transformer 203.
In FIG. 2, integrated circuit 202 provides transistors 206 and 207 for turning on and turning off transistor 201. When the voltage at terminal 208 rises above the voltage at terminal 231 by approximately two volts, transistor 206 turns on and charges the gate terminal of MOSFET 201, thereby turning on MOSFET 201. When the voltage at terminal 208 falls below terminal 233 by approximately 1 volt, transistor 207 is turned on which, in turn, triggers silicon-controlled rectifier (SCR) 205 to turn on also. SCR 205 rapidly discharges the charge at the gate terminal of MOSFET 201, and stops current from flowing into the base terminal of transistor 206, to shut off the current in MOSFET 201 and transistor 206. SCR 205 is shut off when its current falls below approximately 10 ma. At that point, resistors 209 and 230 discharge the remaining charge in the gate terminal of MOSFET 201.
One drawback of circuit 200 is the false triggering of SCR 205 under a number of circumstances. For example, if certain minimum off-time requirements are not met, a very fast negative going voltage at terminal 208 can trigger SCR 205 to turn on. Also, a rapid increase in the voltage at terminal 231 can trigger SCR 205 to turn on. When SCR 205 is triggered on, the current in MOSFET 201 is prematurely shut off. Such premature shut off of current in MOSFET 201 is undesirable, since the amount of current delivered by MOSFET 201 may be insufficient to achieve defibrillation.
Also, the large Miller capacitances in an output transistor, such as MOSFET 201 or an IGBT, can cause the output transistor to inadvertently switch on as a result of a sudden surge in drain-to-gate voltage at the output transistor. The resulting current in the output transistor can disrupt its proper operation and may lead to permanent damage to the output transistor.
Furthermore, the high current MOSFET driver circuit 202 turns off output transistor 201 in response to the voltage at input node 208 going low. Hence, the secondary winding of transformer 203 is required to sustain a positive pulse over the desired on-time for transistor 201. To achieve the desired on-time, the size of transformer 203 is constrained to be undesirably large.